The present invention relates to an apparatus according to the preamble of claim I for fabrication of thin films.
In an apparatus disclosed herein, a substrate placed in a reaction space is subjected to alternate surface reactions of at least two different reactants suitable for fabricating a thin film. The vapor-phase reactants are fed in a repetitive and alternating manner each at a time from its own supply into a reaction space, wherein they are brought to react with the surface of a substrate in order to produce a solid-state thin film product on the substrate. Reaction products not adhering to the substrate and possible excess reactants are removed in gas phase from the reaction space.
Conventionally, thin films are grown out using vacuum evaporation deposition, Molecular Beam Epitaxy (MBE) and other similar vacuum deposition techniques, different variants of Chemical Vapor Deposition (CVD)(including low-pressure and metallo-organic CVD and plasma-enhanced CVD) or, alternatively, the above-mentioned deposition process based on alternate surface reactions, known in the art as the Atomic Layer Epitaxy, shortly ALE, or Atomic Layer Deposition, (ALO). In this description, the term xe2x80x9cALExe2x80x9d will be used, In the MBE and CVD processes, besides other variables, the thin film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform surface smoothness of the thin films manufactured using these methods, the concentrations and reactivities of the starting materials must be kept equal on one side of the substrate. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, the possibility of mutual reactions between the reagents is always imminent. Herein arises a risk of microparticle formation already in the infeed lines of the gaseous reactants. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. However, the occurrence of premature reactions in MBE and CVD reactors can be avoided, e.g., by heating the reactants not earlier than or only at the substrates. In addition to heating, the desired reaction can be initiated with the help of, e.g., plasma or other similar activating means.
In MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the thin film growth rate in the ALE process is controlled by the substrate surface properties, rather than by the concentrations or other qualities of the starting material inflows. In the ALE process, the only prerequisite is that the starting material is provided in a sufficient concentration for film growth on the substrate.
The ALE method is described, e.g., in FI Pat. Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI Patents Nos. 97,730, 97,73 and 100,409 are disclosed some apparatus constructions suited for implementing the method. Equipment for thin film deposition are further described in publications Material Science Report 4(7), 1989, p. 261, and Tyhjixc3x6tekniikka (title in English: Vacuum Techniques), ISBN 951-794-422-5, pp. 253-261.
In the ALE deposition method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon. According to the conventional techniques known from FI Patent Specification No. 57,975, the saturation step is followed by a protective gas pulse forming a diffusion barrier that sweeps away the excess starting material and the gaseous reaction products from the substrate. The successive pulses of different starting materials and the protective gas pulses forming diffusion barriers that separate the successive starting materials pulses from each other accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. To the function of the process it is irrelevant whether they are the gases or the substrates that are kept in motion, but rather, it is imperative that the different starting materials of the successive reaction steps are separated from each other and arranged to impinge on the substrate alternately.
Most vacuum evaporators operate on the so-called xe2x80x9csingle-shotxe2x80x9d principle hereby, a vaporized atom or molecule can impinge on the substrate only once. If no reaction with the substrate surface occurs, the atom or molecule is rebound or re-vaporized so as to hit the apparatus walls or the vacuum pump undergoing condensation therein. In hot-wall reactors, an atom or molecule impinging on the reactor wall or the substrate may become re-vaporized and thus undergoing repeated impingements on the substrate surface. When applied to ALE reactors, this xe2x80x9cmulti-shotxe2x80x9d principle can offer a number of benefits including improved efficiency of material consumption.
In practice, the xe2x80x9cmulti-shotxe2x80x9d type ALE reactors are provided with a reactor chamber structure comprised of a plurality of adjacently or superimposedly stacked modular elements of which at least some are identical to each other and by milling, for instance, have reaction chambers made thereto with suitable cutouts and openings serving as the inlet and outlet channels. Alternatively, the substrates can be placed in an exposed manner in the interior of the vacuum vessel acting as the reaction space. In both arrangements, the reactor must be pressurized in conjunction with the substrate load/unload step.
In the fabrication of thin-film structures, it is conventional that the reactors are preferably run under constant process conditions stabilized in respect to the process temperature, operating pressure as well as for other process parameters. The goal herein is to prevent the attack of foreign particles and chemical impurities from the environment on the substrates and to avoid thermal cycling of the reactors that is a time-consuming step and may deteriorate the process reliability. In practice, these problems are overcome by using a separate substrate load/transfer chamber. The substrate loading chamber communicates with the reactors and is kept under a constant vacuum. The load and unload steps of the substrates are performed so that both the reactor and the loading chamber are taken to a vacuum, after which the valve (such as a gate valve) separating the two from each other is opened, whereby a robotic arm constructed into the loading chamber removes a processed substrate from the reaction chamber and loads a new substrate. Subsequently, the valve is closed and the process may start after the substrate and the reactor have attained their nominal process values. On the other side, the processed substrate is transferred via another controllable valve from the loading chamber to a vacuumized load lock, after which the load lock valve is closed. Next, the load lock may be pressurized, after which the substrate can be removed from the equipment via a third valve opening into the room space. Respectively, the next substrate to be processed can be transferred via the loading chamber into the reactor.
In conventional constructions, the substrate is placed on a heater so that the robotic arm can move the substrate to a desired point in the interior of the reactor, after which the substrate is elevated typically with the help of three pins directly upward for the duration of the robotic arm withdrawal. Next, the substrate is lowered onto a heatable susceptor platform by lowering said pins below the surface level of said susceptor, whereby the substrate remains resting in a good thermal contact with the susceptor.
In the above-described types of reactors, the gas flow enters the reaction space via a xe2x80x9cshower headxe2x80x9d located above the substrate so as to distribute the gas over the hot substrate, whereby the desired surface reaction can take place and form a desired type of thin-film layer on the substrate surface. If used in an ALE-reactor, however, this type of infeed technique would require that, at the beginning and end of each reactant infeed pulse, a period of a duration generally equal to that of the reactant pulse length would become indispensable in order to allow for the homogenization of the gas concentration and flush-out of the previous gas pulse. In practice, this would lead to the mixing of the reactant vapors with each other, whereby the ALE mode of film growth would actually turn into a CVD process. By the same token, the process is hampered by a slow throughput, poor material utilization efficiency and/or large thickness variations.
Furthermore, the walls of the vacuum vessel would respectively become covered with condensed layers of starting materials, and the consequences particularly in conjunction with the use of solid-state sources would be the same as those discussed above.
The reason for running the ALE process in a batch mode is because the ALE method is relatively slow as compared with many other types of thin-film growth techniques. Batch processing, however is capable of bringing the total growth time per substrate to a competitive level. For the same goal, also the substrate sizes have been made larger.
The ALE method also can be utilized for depositing composite layer structures, whereby a single run can be employed for making a plurality of different film structures in a single batch. Thus, also the processing time per fabricated unit can be reduced.
The large stack assemblies required in batch processes are typically put together in some auxiliary space, after which they must be transferred as compact units into the interior of the opened reactor. Typically, the bake-out heating of the reactor chamber structures takes a few hours (1-4 h) followed by the processing step (taking about 2-4 h to a thickness of 300 nm Al2O3), and the cooling lasts up to tens of hours depending on the size of the reactor construction. Furthermore, a certain time must be counted for the dismantling and reassembly of the reactor chamber.
The proportion of the processing time to the work time required by the other processing steps becomes the more disadvantageous the thinner are the thin films (e.g., in the range 1-50 nm) to be grown, whereby the duration of the actual deposition step may last from one to several minutes only. Then, an overwhelming portion of the total processing time in respect to actual processing time is used for heating/cooling the reactor chamber structure, pressurization of the reactor, dismantling/reassembly of the reactor chamber, bringing the system to vacuum and reheating the same.
It is an object of the present invention to overcome the drawbacks of the prior-art technology and to provide an entirely novel apparatus for growing thin films of homogeneous quality using the ALE method in a commercial scale. It is a particular object of the invention to provide an apparatus construction suitable for fabricating very thin films under such circumstances in which the reactor and the structure forming the reactor chamber are all the time kept under stabilized process conditions, whereby the heating, pressurizing and vacuum pumping cycles are performed in respect Lo the substrates alone. It is a further object to provide a reactor design that allows single wafer processing, using the advantages of the previous ALE reactors, viz. minimal reaction volume, aerodynamic design for smooth passing of pulses without (dead volumes.
The goal of the invention is achieved by virtue of a novel concept in which the benefits of a reactor chamber structure and those of a cold-wall ALE reactor are combined with those of reactor equipped with an internal loading chamber through designing reaction chamber construction such that can be opened and closed in the interior of the reactor for the substrate load/unload steps. The reactor volume is so small that there is not enough space for wafer handling. Therefore, the reaction chamber needs to be opened, not by a valve but by taking parts of the reactor apart, for wafer transfer. To prevent exposure of the reaction chamber to the ambient, a second chamber is built around the reaction chamber.
Accordingly, the reaction chamber construction according to the invention comprises at least one part movable with respect to the remaining part of the reaction chamber and adapted to be sealably closable against said remaining part of the reaction chamber such that a substrate can be loaded/unloaded into and out of the reaction chamber. According to one embodiment which will be described in more detail below, the reactor chamber comprises at least two basic parts, namely a stationary base part of the reactor structure and a movable part of said structure, the latter being adapted to be sealably closable against said base part, whereby the reaction space remaining between said base part and said movable part attains the characteristic shape required from such a volume. One combination element formed by the base part and the movable part of the reaction chamber structure acts as a substrate support platform, or base, on which a substrate can be positioned by means of a robotic arm or by the movements of a given part of said arm. However, the basic idea is to have reactor chamber parts which are movable with respect to each other. e.g. one horizontally, the other vertically, or both vertically.
The invention offers significant benefits. Accordingly, the construction according to the invention counteracts to the contamination of the intermediary spaces of the reactor, whereby also the cleanliness of the loading chamber is improved, the contamination risk of substrates reduced and the base pressure can be pumped down at a faster rate. The commercial processing of single substrates facilitated by the apparatus according to the invention offers the further benefit that each substrate can be processed in a tailored manner if so desired. Moreover, the ALE process need not anymore be run in a batch mode if the other production units are operated in a continuous mode.
The apparatus according to the invention is also applicable to the fabrication of different types of thin films including those of the semiconductor variety, as well as thin-film structures of display units by means of the ALE method.